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Study Objectives: This study aimed to investigate the mechanisms of a combined brief cognitive behavioral plus bright light therapy (CBT-I+Light) in women receiving chemotherapy. Methods: Women (N = 101) were randomly assigned to CBT-I+Light or treatment as usual plus relaxation audios (TAU+). Participants completed sleep diaries and wore an actigraph during the 6-week intervention period. Patient-reported outcomes were assessed at baseline, mid-point (week 3), and later (week 6). Cognitive (i.e., dysfunctional sleep beliefs, pre-sleep cognitions, and arousal) and behavioral (i.e., time in bed awake and day-to-day out-of-bedtime variability) mechanisms were examined. Results: Cognitively, both groups declined significantly in overall dysfunctional sleep beliefs from pre- to post-intervention (both p< .04); however, they did not differ on sleep-related beliefs nor pre-sleep cognitions and arousal at post-intervention (both p> .50). Dysfunctional beliefs sleep expectations subscale was lower in CBT-I+Light versus TAU+ (p= .01). Behaviorally, CBT-I+Light reported less overall time in bed awake after the start of the intervention (p< .05) and significantly less time in bed during the morning until the final week of the intervention period. Out-of-bedtime day-to-day variability was lower in the CBT-+Light vs TAU+ at the final intervention day. Conclusion: Mechanisms of CBT-I+Light during chemotherapy remain to be shown. Our results suggest that changes in behavioral mechanisms may be associated with sleep improvements within this cohort. Future studies should assess the role of additional mechanisms (e.g., sleep effort) within larger samples. Whilst intervention brevity is important, more potent interventions may be required to achieve robust changes in target mechanisms.